i. Technical Field
The present invention relates generally to the field of cellulose fiber refiners used in the papermaking industry, and relates more specifically to an improved design for grooves in refiner plates particularly useful for reducing the length of long fibers without overly treating short fibers in the slurry.
ii. Technical Background
A paper sheet is comprised of a network of cellulose fibers randomly distributed and bonded together as water is removed from a slurry of pulp during formation of the sheet. Many of the formation characteristics of the pulp and physical qualities of the resulting sheet are dependent on the strength of the fibers themselves, the number and strength of bonds formed between adjacent fibers, and other physical characteristics of the fibers. Pulp refiners have been used to mechanically treat the fibers, such as to fibrillate or fray the ends and walls of the fibers, thus increasing surface area and increasing bonding sites between fibers. Flexure of the fibers during the refining process increases flexibility by breaking bonds between concentric layers in the individual fibers.
Different wood species exhibit different fiber characteristics and sheet formation qualities. Fiber length and cell wall thickness have a significant effect on the properties of paper formed. The relationship between tear strength, for example, and the length of fibers used in the sheet is virtually directly proportional. Typically, hard woods provide a higher percentage of shorter fibers. Soft woods, on the other hand, are composed of higher percentages of long tapering cells, and certain soft wood species, such as Douglas Fir and Redwood, have relatively thick fibers which tend to produce sheets of high tear resistance but low burst and tensile strength. Papermakers attempt to control various fiber characteristics to achieve strength as well as surface characteristics of the resulting sheet. Whereas long fibers tend to yield stronger pulps, certain surface characteristics, such as smoothness, opacity and the like, are developed by the short fibers.
For these reasons, fibers developed differently, or from differing species, often are mixed. In some regions, such as the West Coast of the United States, hard woods are in short supply while there are abundant supplies of soft woods, such as Douglas Fir and Redwood. Pulps from many of these soft wood species exhibit extreme formation problems, such as flocculation, which is the agglomeration or clumping of fibers, preventing even fiber distribution.
It is sometimes beneficial when using soft woods to treat the long fibers by shortening them without significant treatment to already present short fibers. Conventional long fiber treatment has been attempted using high intensity refining including the use of coarse refining plates run at slow speed using low consistency pulp. High intensity refiners of this type are difficult to operate, and exhibit rapid plate wear resulting in high operation costs. In conventional refining of this type, any significant long fiber treatment has resulted in significant fines generation and an accompanying drop in freeness. While certain levels of changes in freeness and fines percentages can be tolerated, unfortunately, any appreciable effect on the long fibers has been accompanied by excessive fines generation and drops in freeness.